Knee prostheses have been successfully implanted in older patients suffering from arthritis. Older patients are defined as persons approximately 65 years and over. Generally, a knee prosthesis comprises three components: the kneecap or patella, the lower end of the thigh bone or distal femur, and the upper end of the shin bone or proximal tibia. In current knee prostheses the patella is usually a plastic dome of ultra-high molecular weight polyethylene (UHMWPE) with or without a metal backing. A total prosthetic patella is generally impractical due to the impossibility of attaching viable ligaments thereto. The femur is without exception, made of a solid biocompatible metal alloy (such as cobalt chromium alloy, stainless steel or titanium alloy), and the proximal tibial prosthesis is made of UHMWPE with or without a metal backing.
In older sedate patients the existing prostheses may relieve arthritic pain and restore motion to a large extent, although some problems such as loosening of the prosthesis from the bone, poor or improper tracking of the patella, less than full normal extension and flexion and breakdown of materials may exist. When, however, the existing implants are used for younger patients, who may be heavyset or who are physically active or engaged in heavy work, problems arise, such as premature rapid wear of the UHMWPE and breakage of the remaining patellar bone in addition to the above-mentioned problems. Breakage of the patellar bone is particularly important and is caused by removing much bone and fibrous tissue from the natural patella to accommodate the usual 8- to 10-mm thick, often domed, patellar prostheses, weakening the natural patella by as much as 80% of its original strength. Furthermore, the patella is a relatively small component, compared to the tibia and femoral condyles, and yet the traction forces on the patella can be as high as 1800 lbs (800 N) and the contact forces can reach similar levels. These forces are between 3 and 10 times body weight and the current patellar prostheses have not been designed to accommodate these higher forces in physically active persons. The breakage aforementioned, plus premature wear and fatigue failure of the UHMWPE and the metal backing may thus become acute.